On-board electrical systems of mobile units often use AC generators having a three-phase armature winding on the stator connected in Y or .DELTA. configuration. Rotation of a field relative to the armature by a suitable power source induces a three-phase current in the armature, usually in the form of stator windings. The rotor commonly carries an exciter winding which receives exciter current via two slip-rings. In less frequent cases, the exciter winding also is stationary and the changing magnetic field for inducing alternating current in the armature stator windings is produced by means of specially shaped rotating components, e.g. a claw pole structure.
Because on-board electrical systems of motor vehicles are usually direct-current systems, the three-phase alternating currents are rectified by a rectifier bridge commonly including three so-called negative diodes and three positive diodes. Three complementary exciter diodes may also be present to produce an exciter current which is regulated by a regulator in response to a given load and rpm of the three-phase generator. The regulated current is supplied to the exciter winding.
Some generators have a compound winding which is normally so connected that the entire load current flows through it. This compound winding carries a supplementary exciter current and provides a load current dependent excitation.
A load current dependent voltage drop occurs across the compound winding, a result of being connected directly to the positive on-board B+ terminal. To keep the voltage drop low, it is therefore necessary to wind the compound winding with large diameter wire which entails substantial expense. In addition, the unavoidable voltage drop of the compound winding is not compensated by the regulator, because the exciter diodes feeding the regulator, and connected to the D+ terminal of generator system, are connected directly to the armature phase windings of the stator, resulting in undesirable regulation characteristics of the generator.